Like conventional combustion engine systems, fuel cell systems include a coolant system, which circulates a cooling fluid through a fuel cell stack, for example, through bipolar plates of the fuel cell stack, to control the operating temperature of the fuel cell within a desired temperature range. A radiator removes heat from the coolant. The radiator is connected to a coolant tank reservoir. The radiator may have a pressure valve which opens to allow coolant in the radiator to flow over into the coolant tank reservoir if a certain pressure in the radiator is reached. However, some radiators may not include such a pressure valve. Hydrogen or other gases used as a reactant gas in the fuel cell may make its way into the cooling system and accumulate in the coolant tank reservoir. The cooling tank reservoir may also include a valve designed to release the gases, such as hydrogen accumulated in the headspace of the coolant tank reservoir at a certain pressure.